1. Field of the Invention
This invention relates generally to lightning protection devices and methods, and more particularly to lightning air terminals and a method of design and application of such terminals.
2. Description of the Related Art
In the field of practical lightning protection, there is a wide spectrum of technologies currently being used. At one end of this spectrum there are the air terminals claiming enhanced or more consistent performance. Whether these terminals enhance or retard corona development, or whether they are blunt or sharp, they have been broadly categorized under a generic term “ESE” meaning Early Streamer Emission.
In the center of the spectrum there is the conventional practice widely specified in various Standards. This practice currently uses an “electrogeometric” model known as the “rolling sphere method” which was adapted from lightning strike measurements relevant to the electric power transmission industry. Transmission lines are remarkable for their essentially two dimensional aspect and uniformity of height and conductor diameter. Hence, the rolling sphere method could take little or no account of variations in electric field enhancement, such as is the case for different air terminal configurations and point of placement on structures. Nevertheless, the method was adopted for direct application to the protection of three dimensional and other complex, geometrical structures.
Within the Standards there is permitted a widely divergent practice. This may vary from a small number of, for example, six-meter-high Franklin rods, to much shorter terminals, sometimes called finals, spaced at closer intervals. There are also systems with no vertical terminals, sometimes called Faraday systems, which include conductors laid horizontally on exposed surfaces.
At the other end of the spectrum are the systems that claim to prevent lightning attachment by the use of arrays of sharp points designed to produce abundant corona. The corona is claimed to discourage upward leader development and discharge the thundercloud, causing the lightning to strike elsewhere.
While none of the above techniques offer perfection, there is room to improve the performance of air terminals and their location through a better understanding of the attachment process. What follows is one explanation of the attachment process, although it will be appreciated that actual attachment processes may deviate from below-described process.
There are four phases in the attachment process of lightning to a ground point. The first is the quasi-static phase where electrical fields build below a storm cloud over some tens of minutes. These fields cause ground objects to be electrically stressed and, depending on their height and geometric shape, they will emit corona. In the case of a negative cloud base, this corona is in the form of positive ions that create a space charge volume above the object.
In the longer term, these positive ions, which in reality are molecules of air, ascend with typical velocities of 1 ms−1 in fields of 10 kVm−1. They create non-linearity in the field to heights of several hundred meters. Thus, the electric field strength observed at ground becomes modified before any dynamic event occurs. Typical ambient fields of 50 kVm−1 have been recorded as reducing to values below 5 kVm−1 near ground.
The second phase relates to the approach of a downward leader, a filament discharge with average velocity of approximately 105 ms−1 when pauses of 20-50 μs (microseconds) are taken into account. The inter-pause or step velocities can exceed 106 ms−1. This conveyance of charge toward ground causes a rapid increase in the field strength observed by ground points. There is very small initial change in the ground observed electric field strength when the leader is at high altitude, but with near approach, values will be escalating at a typical rate of 109 Vm−1s−1.
The third phase is when electric field strength observed by a ground point reaches the critical value to create avalanche breakdown near the tip of ground points. This process commences with an initial corona burst and the development of a “streamer zone”. The streamer zone initially extends from the tip of the object out to, typically, 0.5-1 meter from the object and is comprised of many filamentary discharges called “streamers”. One of these streamers may eventually become thermalised, forming a leader stem. Provided the field strength is sufficient, a new streamer zone then develops ahead of the leader. This process repeats as the leader discharge propagates upwards, toward the approaching downward lightning leader. Electric field computations can be made to quantify the point at which these processes will occur during this third phase, based on the height and ground electrode curvature amongst other things. Streamer development fields may also be determined in the laboratory, but it has proven difficult to perform laboratory experiments to readily model the field decay from the surface to the first few meters above a terminal, and eventually to the “ambient” value. The “ambient” field is defined as the unperturbed electric field, i.e., that which would exist in the absence of the object. There is a minimum value of the field required to convert a streamer into a propagating upward leader.
The fourth phase is the continuing propagation of the upward leader. Once the stem of an upward leader is formed, an electric field of 400-500 kVm−1 is required in the streamer zone to provide the necessary energy to continue propagation.
Embedded within the above four phases is another spectrum based on the strength of electric field to cause breakdown of air, and the electric field strength required for streamer-leader inception. The former value has a nominal value of 3 MVm−1. The latter value falls within the range 400-500 kVm−1 for positive streamer-leaders (the most common polarity for upward discharges). Of course, in nature these values will never be exact, with some parameters varying by orders of magnitude.
There is a wide variation in geometric shape of ground points which range from sharp points to flat horizontal surfaces. At one end of the geometric shape spectrum is the pointed Franklin rod. Should this rod produce a field intensification of 1000:1, then 3 MVm−1 at the tip is reached when the ambient field is only 3 kVm−1. No streamer development or propagation is possible in such low ambient fields but a continuing corona emission will provide an ascending space charge of ionized air molecules in periods long before the initiation and descent of a downward leader.
As the center of this spectrum is approached, the field intensification progressively reduces. The center is reached when, for example, a value of 6:1 is achieved over a critical range for the particular terminal configuration. This center of the spectrum would typically be a “blunt” rod which has a rounded upper surface of a given radius. In this case, the field strength at the tip of the rod reaches a corona emission level of 3 MVm−1 at the time when the ambient field reaches the leader propagation level of 500 kVm−1.
At the other extreme of this spectrum is a flat surface with unity field intensification. Hence, the downward leader needs to approach very closely to produce 3 MVm−1 at the surface. In this case, air ionization, corona emission and breakdown occur simultaneously.
This spectrum leads to a number of conclusions, namely, that an elevated sharp point becomes unnecessarily active too early in the process, by producing field-reducing corona along with space charge. This blanket of charged particles lying above the grounded point will, to a varying extent, mask the field due to the approaching downward leader. The result is that the downward leader must approach much closer in order to force the creation of an upward leader. It has been discovered that a rounded surface will provide a better performance by minimizing pre-discharge corona and, by suitable radius or diameter dimension and initiate streamers only when the ambient field can support their conversion into a stable leader.
Hereafter, three different types of air terminals will be referred to, viz.:
(I) A fully grounded conductor as specified in various Standards, i.e., a Franklin rod which is a long cylindrical conductor with a sharp, conical tip, the shorter final version, or the rodless system of copper tapes commonly known as the Faraday system. Henceforth, these types of air terminal shall be referred to as “conventional passive”.
(II) A particular type of air terminal comprising a curved conductor, typically a sphere, placed on a conductive rod. The radius of curvature and overall height of this air terminal may be dimensioned according to the method described in Gumley, U.S. Pat. No. 6,320,119, which is herein incorporated by reference in its entirety. Hereafter, this type of air terminal shall be referred to as “RFI passive”, RFI being the acronym for “reduced field intensification”.
(III) A particular type of curved surface air terminal comprising one or more insulated components which result in a triggering arc to enhance the initiation of the lightning attachment process; henceforth, this type of air terminal shall be referred to as “RFI triggering”.
One example of a prior Type III system is that disclosed in Gumley, U.S. Pat. No. 4,760,213. Such Type III terminals are widely sold under the trademark DYNASPHERE™ by ERICO, Inc. of Solon, Ohio, USA.
The DYNASPHERE™ terminal utilizes a generally spherical or ellipsoidal curved surface electrode which is connected to the grounded central conductor via a high impedance current drain. An annular air gap is provided between the top of the generally spherical surface and the top of the central grounded conductor. Such lightning air terminals have a number of parameters such as the size and shape of the spherical surface, the size of the air gap, the shape of the tip of the central grounded conductor, the height of the terminal above the structure to be protected, and the location of the air terminal on the structure. One primary parameter is known as the “electric field intensification factor” which is derived from the height and curvature of the curved surface electrode. These factors have never before been defined in relation to practical lightning protection systems.
U.S. Pat. No. 6,320,119 describes: (i) certain improvements of Type I lightning air terminals, viz. the Type II terminals, (ii) certain improvements in the Type III system described in U.S. Pat. No. 4,760,213, and (iii) a method of design and application of the Type II & III air terminals. Terminals described in U.S. Pat. No. 6,320,119 are also available from ERICO, Inc.
It will be appreciated that further improvements may be desirable for lightning protection systems and methods.